Morphology and plasmonic properties of self-organized arrays of ...

102 CHAPTER 6. COMPOSITE MEDIA BASED ON AU/LIF ARRAYS0.250.20T modeL mode0.350.300.25T modeL modeR S0.15R S0.200.150.100.10400 600 800 1000a. [nm]b.400600 [nm]8001000Figure 6.5: Experimental (empty circles) and computed (continuous lines) R S spectra,with plane of incidence parallel (black curves) and perpendicular (red curves) the LiFridges. Left panel: 2D arrays of gold NPs on nanopatterned LiF(110) (Λ = 35 nm,t Au ≈ 5 nm, AFM data in fig. 6.2(a, b)). Right panel: same system after the depositionof a monolayer of Fe 3 O 4 /OA NPs (AFM data in fig. 6.2(c)).monolayer of deposited NPs, the first one was to increase the thickness of the effectivelayer by an amount equal to the mean diameter of the NPs, i.e. ≈ 17 nm. The secondcorrection was to rewrite the dielectric constant ε h of the host including the contributionof the magnetite NPs; we considered the gold NPs embedded in an effective host composedat 50% by LiF (with dielectric constant ε s ) and at 50% by a mixing of air and bulkmagnetite (with dielectric constant ε Fe3O 4, assuming that the dielectric constant of Fe 3 O 4nanoparticles does not differ from the bulk):ε h = 1 2 ε s + 1 2 (f ε Fe 3O 4+(1−f)ε air ) (6.1)where ε air = 1 and f is the magnetite filling factor. The latter is calculated by simplegeometrical considerations: assuming the Fe 3 O 4 /OA nanoparticles arranged on a squarelattice, and in contact with one another, we find one particle per volume of d 3 hydro (d hydrois the hydrodynamic size), while the volume of the magnetite core is 4π/3(d core /2) 3 (d coreis the size of the NPs core); the filling factor f is given by the ratio of the two volumesand, substituting the values obtained from TEM and DLS measurements, results f ≈ 0.3.The calculated R S spectra for the Au NPs array after the Fe 3 O 4 /OA deposition arereported in fig. 6.5(b). Despite the simplifications in treating the magnetite NPs layer,which probably determined the overestimation of the absolute values of reflectivity, thepositions and widths of the LSPs were reproduced in good agreement with the experiments,resulting of λ ∗ L = 645 nm and Γ∗ L = 223 nm for the L mode and λ∗ T = 576 nmand Γ ∗ T = 145 nm for the T mode. In particular, this also supports the consideration thatonly one monolayer of Fe 3 O 4 /OA NPs has been deposited.Inconclusion,wehaveshownthattheself-organizedAu/LiFsystemscouldbefruitfullyexploited as templates for the fabrication of more complex composite structures, that canatleastpartlyretainthemorphologicalcharacteristicsoftheoriginalsystemanditsopticalresponse, adding to these novel functionalities, like magnetic response in this specific case.